1. Technical Field
The present invention relates to an image sensor, more specifically to an apparatus and a method of providing a sharper image by preventing the deterioration of the image caused by the difference in dynamic ranges of a center part and a surrounding part of the image photographed by an image sensor or the difference of lens resolution.
2. Description of the Related Art
An image sensor refers to a semiconductor device converting an optical image to electric signals. Portable apparatuses (e.g. digital cameras and mobile communication terminals) having an image sensor are now developed and sold. The image sensor consists of arrays of small photo diodes, which are called pixels or photosites. The pixels themselves typically do not extract color from light. The pixels merely convert photos, provided from a wide spectrum band, to electrons. To write color images by using a single sensor, the sensor is filtered such that different pixels can receive different color light. This type of sensor is known as a color filter array (CFA). The different color filters intersect the sensor and are arrayed in a predetermined pattern.
In addition to color filters, the image sensors are equipped with various image filters. Most filters are designed to evenly apply a preset filter coefficient or filter type to an entire image frame. Although an image has different properties for each area, the same settings are applied entirely to one image frame, malting it difficult to effectively express the features of the image.
FIG. 1 illustrates an image of an image sensor and an area thereof having different properties; FIG. 2 illustrates properties of an image for each area; and FIG. 3 illustrates a method of compensating the properties for each area of an image.
Referring to FIG. 1, the properties of an image 100 are typically changed in the direction from a center pixel 110 of a center part toward each of edge pixels 120a, 120b, 120c and 120d (hereinafter, collectively referred to as 120). In other words, portions having similar properties can be distinguished by each of concentric rings 130a, 130b, 130c and 130d. 
FIG. 2 shows the brightness, among various properties, according to the position of a pixel in the image 100. A first curve 210 indicates the maximum brightness for each pixel, and a second curve 220 indicates the minimum brightness for each pixel. The first curve 210 and the second curve 220 are brightest in the center pixel 110 and darkest in the edge pixel 120. The first curve 210 and the second curve 220 become less bright as the pixel position is changed from the center pixel to the edge pixel.
If a dynamic range D1 of the center pixel 110 is compared with dynamic ranges D1 and D2 of the edge pixels 120, the dynamic range D1 of the center pixel 110 is wider. Here, the dynamic range refers to the difference between the darkest brightness and the brightest brightness that can be expressed in a pixel. In other words, the wide dynamic range leads to the high resolution, and the narrow dynamic range leads to the low resolution.
If the dynamic range D1 of the center pixel 110 is compared with dynamic ranges D1 and D2 of the edge pixels 120, the difference of up to 30 to 40% occurs depending on the lens property of the image sensor. Also, the image becomes more burred in a surrounding part having the edge pixels 120 than in a center part having the center pixel 110. This is because a beam of light passing through a lens is distorted and the focal point is not uniform. As a result, the edge is expanded and the sharpness is lowered.
For acquiring the dynamic ranges, referring to FIG. 3, the dynamic ranges of the entire image are required to be smoothed based on the dynamic range of the center pixel 110 (referring to a first arrow 310 and a second arrow 320). Accordingly, the dynamic range D2 of the surrounding part (having the edge pixels 120) is changed to D2′. For this, a certain ratio of gain is multiplied or a device performing a lens shading compensation function is used in order to compensate the dynamic ranges of the whole image.
However, in this case, the noise component is also amplified in the surrounding part having the edge pixels 120 due to the expansion of the dynamic ranges. Even if the edge is detected and emphasized, the noise component is also emphasized. Accordingly, the quality of the image 100 is deteriorated. Further, the edge level is smaller in the surrounding part having the edge pixels 120, to thereby lower the sharpness in the surrounding parts of the image 100 and deteriorate the quality of the image 100.